Field effect transistors are broadly classified as depletion mode or enhancement mode transistors based on whether the transistor is in ON state or in OFF state at zero gate-to-source voltage. Enhancement-mode field effect transistors are commonly used in most circuits. Enhancement-mode field effect transistors are normally OFF when gate-to-source voltage is zero and are turned to an ON state by moving the gate relative to the source voltage towards the drain voltage, e.g., to a positive potential for a transistor with an n-type conductive channel. In biasing the gate, the conduction of the channel is enhanced and hence these devices are usually referred to as enhancement-mode devices.
In contrast, depletion-mode field effect transistors are normally ON when gate-to-source voltage is at zero and are turned to OFF state by moving the gate relative to the source voltage, e.g., to a negative potential for a transistor with an n-type conductive channel. In the OFF state, the conductive channel is depleted and hence these devices are usually referred to as depletion-mode devices.
High electron mobility transistors (HEMTs), which are also referred to as heterostructure FETs (HFETs) or modulation-doped FETs (MODFETs), are one type of depletion mode device because a current flows between source and drain terminals of the device when the gate electrode is grounded. HEMTs are typically transistors comprising III-V materials and have a two dimensional electron gas layer in an unbiased state. That is, a thin electrically conductive channel (inversion layer) exists between the source and drain terminals prior to application of any gate voltage. As such, the device is commonly referred to as being normally-on device.